"We're both preparing for a long, drawn-out affair," Plouffe said today. "If it goes through June, it goes through June."
"We are looking at a fight that is going to go, as we've said, well beyond tomorrow, possibly decided in March, possibly decided in April, possibly not decided until the convention," Clinton's communications director Howard Wolfson said Monday.
At stake Tuesday were 1,681 convention delegates — 87 percent of the total needed to clinch the Democratic nomination.
Obama and Clinton are poised to split delegates in contests across the country because Democratic Party rules allocate most of the delegates proportionately. The delegates will be awarded to the contenders based on their shares of the popular vote.
Tuesday afternoon, Obama attempted to lower expectations for tonight's Super Tuesday results as he cast his balllot blocks from his South Side Chicago home.
"I still think Sen. Clinton is the favorite. She had 20-30-point leads in many of these states, we've been closing some ground," Obama said inside an elementary school. "My guess is we'll have a good night and we'll probably end up having a split decision."
Clinton and her husband, former President Bill Clinton, cast their votes early this morning in Chappaqua, N.Y.
"It's a very humbling and overwhelming experience to cast my vote today," she said.
After celebrities like Oprah Winfrey, California first lady Maria Shriver, Caroline Kennedy and Robert DeNiro stumped for Obama over the weekend, Clinton unleashed her own star power today with a robocall starring Oscar-winner Jack Nicholson.
"She never gives up, she never gives in and she's battle-tested," Nicholson says in an automatic call to Californians today. "She'll be a strong commander in chief, she has the experience to deal with the economic challenges we face as a nation today and in the future."
ABC News' Gary Langer, David Wright, Tahman Bradley, Eloise Harper and Sunlen Miller contributed reporting.